1 Field of the Invention
The present invention relates to an installation for fabricating parts by selective melting of powder with the help of a laser beam or of an electron beam.
2 Description of the Related Art
Such an installation enables parts to be fabricated with the help of a method known under the name direct metal laser sintering or electron beam melting.
The method consists in fabricating a part by melting successive layers of powder by means of a laser beam or of an electron beam controlled by a data processor system having recorded therein the three-dimensional coordinates of points of successive layers that are to be made. In practical manner, a vessel having a bottom formed by a plate that is movable in vertical translation has a first layer of powder placed on its bottom with the help of a scraper. The layer then presents a bottom surface corresponding to the top surface of the plate and a top surface onto which the laser beam or the electron beam is directed and over which it is moved. The energy delivered by the beam causes the powder to melt locally, and on solidifying the powder forms a first layer of the metal part.
After this first layer has been made, the plate is lowered through a distance corresponding to the thickness of one layer, and then a second layer of powder is brought by the scraper onto the preceding layer. In the same manner as before, a second layer of the metal part is formed by melting with the help of the laser beam or of the electron beam.
These operations are repeated until the entire part has been fabricated.
The movable plate is generally held by screws to a movable support that is itself moved in vertical translation inside the vessel by actuating an actuator.
It is necessary to guarantee that the top surface of the movable plate is plane throughout fabrication of the part. Unfortunately, the successive passes of the laser beam or of the electron beam give rise to thermal stresses in the movable plate, which can lead to the plate deforming if it is not sufficiently rigid.
In order to have rigidity that is sufficient, the movable plate presents considerable thickness, e.g. thickness of about 60 millimeters (mm) for a plate made of INCONEL 718.
The cost of such a plate is very high, and the assembly comprising the movable plate and the part is also very heavy, thus making it difficult to transport.
Finally, the movable plate has very great thermal inertia and therefore requires long heating periods during the heat treatment applied during fabrication or after fabrication, such as for example heat treatment for detensioning or for relieving stresses.